On 9/29/20 4:29 PM, jimlux wrote:
On 9/29/20 1:24 PM, john@kk9a.com wrote:
With NEC2 I avoid using short wires. If I had a swedged element I
would use the length of the unswedge portion of the larger tube and
for the next wire I would just add the length of the swedge portion of
this tube to the smaller tube that fits inside it. The 1/8 difference
in diameter for the small overlapping area should not make a
difference on HF and I feel that the segments are more equal and that
the model will be reasonably accurate. Does NEC4 model short segments
more accurately?
Yes, there's an improved basis function for the fields along the segment
so it models short segs more accurately. That said, I'd model it as 3
segments: tube from center, slightly larger diameter for overlap,
skinnier tube continuing out.
from nec4userman.pdf
The main electrical consideration is segment length ∆ relative to the
wavelength λ. Generally, ∆ should be less than about 0.1λ at the desired
frequency. Somewhat longer segments may be acceptable on long wires with
no abrupt changes while shorter segments, 0.05λ or less, may be needed
in modeling critical regions of an antenna. The size of the segments
determines the resolution in solving for the current on the model, since
the current is computed at the center of each segment. Earlier versions
of NEC suffered a loss of precision or complete failure of the solution
when very short segments were used, but this problem has been corrected
in NEC–4. The extremely short segments can be used with NEC–4, subject
to limitations related to the wire radius as discussed below.
The wire radius a relative to λ is limited by the approximations used in
the kernel of the electric field integral equation. NEC uses the
thin-wire approximation, neglecting transverse currents on wires and
assuming that the axially directed current is uniformly distributed
around the segment surface. The acceptability of these approximations
depends on both the value of a/λ and the tendency of the excitation to
produce circumferential current or current variation. Unless 2πa/λ is
much less than 1, the validity of these approximations should be considered.
John KK9A
jimlux wrote:
but does the model actually show much difference? One can get way down
in the weeds with this - put a tapered segment in that's 1 cm long, etc.
But if the wavelength is 20 meters, a 1 cm transition is 0.0005
wavelength. I'd worry more about numerical instability than model
accuracy at that point.
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